1. Field of the Invention
The present invention relates to a structure of a semiconductor device provided with a semiconductor circuit made of a semiconductor element such as an insulated gate type transistor, and a method of fabricating the same. Particularly, the invention relates to a structure of a semiconductor device provided with a semiconductor circuit made of a semiconductor element having an LDD structure formed by using organic resin, and a method of fabricating the same. The semiconductor device of the present invention includes not only a element such as a thin film transistor (TFT) or a MOS transistor, but also a display device including a semiconductor circuit constituted by these insulated gate type transistors or an electro-optical device such as an image sensor. In addition, the semiconductor device of the present invention includes also an electronic equipment having these display device and electro-optical device.
2. Description of the Related Art
Attention has been paid to an active matrix type liquid crystal display provided with a pixel matrix circuit and a driver circuit which are constituted by thin film transistor (TFTs) formed over a substrate having an insulating property. The liquid crystal display with a size of about 0.5 to 20 inches is used for a display.
At present, for the purpose of realizing a liquid crystal display enabling high fineness expression, attention has been paid to a TFT having an active layer of a crystalline semiconductor film typified by polysilicon. However, although a TFT having an active layer of a crystalline semiconductor film is superior in operation speed and driving capability as compared with a TFT having an active layer of an amorphous semiconductor film, it has a problem that fluctuation in TFT characteristics is large.
An interface between an active layer and a gate insulating film can be named as one of causes of the occurrence of this fluctuation in TFT characteristics. If this interface is polluted, the TFT characteristics are badly affected. Thus, it is important to clean the interface between the active layer and the insulating film being in contact with the active layer.
At present, a TFT is required to have high mobility, and a crystalline semiconductor film having mobility higher than an amorphous semiconductor film is regarded as the most likely active layer of a TFT. A brief outline of a conventional method of fabricating a TFT will be described below.
First, a gate wiring line is formed on an insulating substrate, a gate insulating film and an amorphous silicon film are stacked thereon, and the amorphous silicon film is subjected to a crystallizing process such as heating or irradiation of laser light, so that the amorphous silicon film is transformed into a polysilicon film. Next, this polysilicon film is patterned into a desired shape to form an active layer. Next, an impurity to give P type or N type conductivity is selectively introduced into the polysilicon film, so that impurity regions which become a source region and a drain region are formed. Subsequently, an interlayer insulating film is deposited, and contact holes to expose portions on the source region and the drain region are formed, and then, a metal film is formed and is patterned, so that metal wiring lines being in contact with the source region and the drain region are formed. In this way, fabricating steps of the TFT are completed.
Like this, in the conventional method, since the insulating film is formed after some steps (for example, crystallizing step, patterning step) are carried out subsequent to the formation of the amorphous semiconductor film, the amorphous semiconductor film is exposed to the atmosphere.
Especially the atmosphere in a clean room contains boron mainly from a HEPA filter (high efficiency particulate air-filter) generally used for cleaning, and when an active layer is exposed to the atmosphere, an indefinite amount of boron is mixed into the active layer. Conventionally, fabrication was made while the active layer was exposed to the atmosphere, and in the case where the SIMS analysis was carried out, an interface (at a main surface side or rear surface side) of an active layer of a TFT had a concentration peak (indicated by a broken line B in FIG. 14) of boron, and its maximum value was 1×1018 atoms/cm3 or more. When boron is mixed into the active layer like this, it becomes difficult to control the concentration of impurities in the active layer, which causes the fluctuation of threshold values of TFTs. In the case where another filter is used, the cost becomes so high that this is not suitable.
Like this, in the conventional method, after a semiconductor film is formed, the surface of the semiconductor film is exposed to the atmosphere, and the semiconductor film which becomes an active layer is polluted by impurities (boron, oxygen, moisture, sodium, etc.) in the atmosphere. Besides, after formation of a gate insulating film, the semiconductor film which becomes the active layer is formed on the gate insulating film which has been exposed to the atmosphere and has been polluted, so that the semiconductor film is polluted by the impurities (boron, oxygen, moisture, sodium, etc.) in the atmosphere. When a semiconductor element, for example, a TFT is fabricated by using the semiconductor film polluted in this way, characteristics of an interface between the active layer, especially a channel formation region and the gate insulating film are degraded, which causes fluctuation and deterioration of electrical characteristics of TFTs. Besides, also in a crystallizing step, the impurities (boron, oxygen, moisture, sodium, etc.) hinder crystallization of the semiconductor film.
Besides, a structure of a thin film transistor provided with an LDD region has been conventionally known. An example of a thin film transistor provided with an LDD region is disclosed in Japanese Patent Application Examined No. Hei. 3-38755 and Japanese Patent Application Laid-open No. Hei. 7-226515. The LDD region relieves the strength of an electric field formed between a channel formation region and a drain region, and serves to lower an OFF current of a transistor and to prevent deterioration. However, a method of fabricating an LDD structure using a conventional technique is complicated, and many steps are required.